Nuclear decay and half-life
Each half-life halves what remains, and alpha, beta and gamma decay change the nucleus in different, predictable ways.
A worked example
A sample with a half-life of 8.0 hours is down to one sixteenth of the original amount after four half-lives, which is 32 hours.
The mistake people actually make
Alpha decay lowers the mass number by 4 and the atomic number by 2. Beta-negative decay leaves the mass number unchanged and raises the atomic number by 1.